With complex hydrodynamical simulations scientists at MPA investigate the detailed impact of supernova explosions on the chemical composition and the thermodynamic properties of the interstellar medium and galactic outflows.

Only a small fraction of gas in the interstellar medium (ISM) of a star-forming galaxy is converted into stars. And less than one percent of all newborn stars are massive enough to die in a supernova explosion after their relatively short life of about 10 million years. Nevertheless, these explosions can have an enormous impact on the ISM and the cosmic evolution of galaxies. With a European team of astrophysicists (the SILCC collaboration), scientists at the Max Planck Institute for Astrophysics used high-resolution supercomputer simulations to investigate the conditions under which supernova explosions can shape the ISM in a galactic disk: realistically and with dense molecular clouds and diffuse neutral and ionized hydrogen for a wide range of scales. In particular, supernovae exploding outside dense molecular clouds can launch powerful gaseous outflows. These outflows change the galactic gas content and might regulate the cosmic evolution of the whole population of star forming galaxies.

A supernova explosion is a most dramatic event at the end of a
massive star’s life. Born in dense molecular clouds, massive stars
evolve rapidly compared to cosmic timescales. At the end of their life –
after several million to a few tens of million years – stars more
massive than eight solar masses do not necessarily explode in the dense
environment they were born. Some have travelled out of their parental
cloud; some explode in low density cavities shaped by their own ionizing
radiation and stellar winds or created by previous supernova explosions
of nearby stars. The environmental density of a supernova explosion is
very important. It determines the explosion impact on the ISM as well as
the whole galaxy. An explosion in a dense environment means that the
energy of the supernova shock is efficiently converted into radiation
and escapes the galaxy. Therefore, the impact on the surrounding ISM is
weak. If the explosion occurs in a low-density environment on the other
hand, less energy is radiated away and the expanding remnant has more
power left for heating and compressing the gas. This can lead to an
enhanced production of hot gas but also of dense structures. The hot gas
is driven out of the galactic disk and sweeps the colder ISM along.

The thermodynamic evolution of supernova remnants, the structure of
their surrounding ISM and the efficiency of outflows are particularly
important for the ecosystem of each individual galaxy. These factors
play a fundamental role in regulating the gas content in each galaxy and
thus the evolution of entire galaxy populations in the Universe.
Explaining outflow properties and connecting simulated data to
observations is therefore key to understanding the formation history of
galaxies.

Together with a European team of experts, scientists at MPA have used
high-resolution supercomputer simulations to investigate the impact of
supernova explosions on the ISM in a galactic disk. For the first time,
the simulations follow not only the kinematics, densities and
temperatures of the gas in the ISM but also the chemical transitions
from ionized gas, over neutral atomic gas to dense molecular gas. The
latter forms mostly on dust grains and can be destroyed by the
interstellar radiation field and in strong shocks like those originating
from supernova remnants.

Assuming a typical supernova rate (up to a few dozen explosions in
one million years) the team has investigated several possible scenarios
for the location of supernova explosions. In Fig. 1 we show the
simulated gas structure of the ISM in a galaxy after 50 million years of
evolution, assuming that all supernovae explode in the densest regions
of the gas where their progenitor stars were born. Explosions in dense
regions inhibit the formation of further cold molecular clouds. The
supernova shells loose their energy quickly and cannot efficiently
accelerate gas or generate a hot, ionized medium. The result is an
interstellar medium mainly made of warm neutral gas with small density
contrasts and very few or no molecular clouds. In this configuration –
which is not in agreement with observations of the ISM – no outflows are
launched.

The situation changes dramatically if supernovae are allowed to
explode in low-density environments. In this case the explosions
generate a more realistic multi-phase medium, as shown in Fig. 2.
Low-density regions are filled with hot ionized gas. Compared to
observations this model is much more realistic. Fig. 3 shows the ISM
structure for these models with supernova explosions in low-density
regions. The ISM is more structured, filled with hot gas and much more
extended out of the plane (in the vertical direction). At the same time
the ISM develops dense molecular clouds, while the hot gas in the low
density regions is expanding and drags diffuse neutral gas with it,
forcing the gas to leave the galactic disk in a clumpy outflow.

Fig. 4
shows more details on the outflows for such a model. The hot ionized gas
is expanding from the disk mid-plane and reaches high velocities of
several hundred kilometers per second. It escapes through low-density
chimneys from the disk.

To understand how supernova explosions impact the evolution of
galaxies, it is crucial to investigate the structural details of the
interstellar medium including the chemical composition, the distribution
of gas, the positions of supernovae and the efficiency with which gas
can escape a galaxy via outflows. The simulations of the SILCC
collaboration are therefore an important step forward in understanding
the regulation of potential star formation and the gas cycle in
star-forming galaxies.